Method and apparatus for fabricating high fin density heatsinks

Abstract: A method of forming a high fin density heatsink (1,3) from a plurality of fins (3) each having a bottom portion of generally bell-bottom shape and a base unit (1) of predetermined thickness having a plurality of grooves (5) therein. The method comprises the steps of placing the plurality of fins (3) loosely in respective ones of the grooves (5), and applying downward and horizontal pressure to the base unit intermediate respective ones of the fins for swaging the base unit so as to urge the fins (3) downwardly into the respective ones of the grooves (5) thereby creating a solid connection between the base unit (1) and the fins (3) with excellent thermal contact therebetween.

Description: FIELD OF THE INVENTION This invention relates in general to the fabrication of heatsinks, and more particularly to a method and apparatus for fabricating a high fin density heatsink from separately extruded fins and a base unit. BACRGROUND OF THE INVENTION It is known in the art to utilize heatsinks for dissipating heat generated by electronic circuits in modern devices. Such well known heatsinks typically comprise a base unit to which the heat generating electronic devices are mounted, and a plurality of fins projecting from the base unit for dissipating the generated heat. It is an object of these heatsinks to maximise the surface area of the fins in order to provide optimum heat transfer from the heatsink to the surrounding atmosphere. It is a further object of well known heatsinks to provide a good thermal contact between the base unit and the fins. In order to achieve the latter mentioned object, according to known prior art methods, heatsinks have been fabricated by metal extrusion through a die which is cut to the required shape specifications such that the base unit and fins are of integral construction. However, as discussed above, efficiency of operation of such heatsinks is regulated by the surface area and amount of metal used versus the length of each fin. The current design limit for prior art extrusion processes in terms of fin thickness to height, is 3:1 or less. Thus, in the event that an extruded fin is excessively thin or excessively high, the pressure of the extruded metal has a tendency to destroy the die. In an effort to overcome the problems of integral extruded heatsinks, and in an effort to provide increased fin density and overall height, certain prior art fabrication methods have used separate extrusion of the base unit and the fins, and subsequent assembly to form the heatsink. For example, U.S. Patent No. 3,261,396 (Trunk) teaches a method of securing corrugated metal fins to a base unit by means of epoxy cement. Unfortunately, the epoxy gluing method disclosed in the Trunk patent results in poor thermal conductivity between the base unit and respective fins. U.S. Patent No. 3,216,496 (Katz) teaches separate fabrication of a bus bar or base unit having, on its flat side, a plurality of slots for receiving separately fabricated fins. The slots extend transversely the length of the bar and are arranged so as to hold the fins in substantially parallel relationship. The fins themselves are strip-shaped and preferably of width equal to that of the bus bar. The fins are joined to the bus bar by means of inserting the fins into the plurality of slots and then swaging the material between adjoining slots into intimate contact with the fins by means of a hydraulic press operating through a knife-edge tool. Unfortunately, the method taught by Katz applies pressure only to the narrow line contacted by the knife-edged tool. This results in the material of the bus bar being pushed to the side of the respective fins which creates an upward pressure tending to lift the fins upwardly from the grooves, thereby leaving an air space at the bottom of the grooves in the bus bar, which allows for air and moisture to enter resulting in corrosion and reducing the thermal contact between the base unit and the fins. Various other patents disclose methods of joining fins to a base unit. For example, United States Patents Nos. 3,312,277 (Chitouras) and 3,280,907 (Hoffman) teach the use of dovetail connections for overcoming extrusion difficulties in fabricating a heat dissipating device with the fins very close together. Additional patents of interest are as follows: U.S. Patents Nos. 941,375 (Loud et al); 2,639,119 (Greenwald); 2,944,326 (Stradthaus et al); 3,077,928 (Nihlen et al); and 4,733,453 (Jacoby). However, none of the above references teach effective means for overcoming the problem inherent in the Katz fabrication method by which the material of the base unit is pushed upwardly between respective ones of the fins thereby causing the fins to lift upwardly from the grooves. SUMMARY OF THE INVENTION According to the present invention, an apparatus and method is provided for fabricating a high fin density heatsink by means of a plurality of rollers for applying pressure on opposite sides of the fin for providing downward and inward swaging against a dovetail joint. In particular, a combination of dovetail fin base and groove is provided with the application of rolling pressure on opposite sides of each fin resulting in vertical and. lateral pressure of the base until material tending to push the fin toward the bottom of the groove in the base. The secure connection between the fins and the base results in good thermal contact therebetween and prevents air and moisture from entering the grooves thereby preventing corrosion and allowing the heatsink to be anodized. Various additional aspects of the invention are as follows: An apparatus for swaging a base unit of predetermined thickness and having a plurality of grooves therein for receiving respective bell-bottom shaped fins to form a high fin density heatsink, said apparatus comprising: a) a planar work surface; b) an axle mounted parallel to said planar work surface; c) a plurality of disks mounted for rotation about said axle and spaced apart a sufficient distance to allow respective ones of said fins to pass therebetween; d) means for adjusting the height of said axle to provide a clearance between the circumference of said disks and said planar work surface which is less than said predetermined thickness of said base unit, such that upon advancing said base unit through said clearance respective ones of said disks apply downward and horizontal pressure to said base unit intermediate respective ones of said fins for swaging said base unit so as to urge said fins downwardly into respective ones of said grooves thereby creating a secure connection between said base unit and said fins with good thermal contact therebetween. A method of forming a high fin density heatsink from a plurality of fins each having a bottom portion of generally bell-bottom shape and a base unit of predetermined thickness having a plurality of grooves therein comprising the steps of: a) placing said plurality of fins loosely in respective ones of said grooves; and b) applying downward and horizontal pressure to said base unit intermediate respective ones of said fins for swaging said base unit so as to urge said fins downwardly into said respective ones of said grooves thereby creating a secure connection between said base unit and said fins with good thermal contact therebetween. BRIEF DESCRIPTION OF THE DRAWINGS A detailed description of the preferred embodiments is provided below with reference to the following drawings, in which: Figure 1 is a perspective view of a heatsink manufactured in accordance with the principles of the prior art; Figure 2 is a partially broken elevation view of the heatsink of Figure 1 following a swaging operation according to the present invention, showing a dovetail connection of base unit having grooves for receiving bell-bottom shaped fins; Figure 3 is a perspective view of an apparatus for swaging the base unit and fins to form the heatsink of Figure 1; Figure 4 is a detail elevation view showing the positioning of a plurality of rollers of the apparatus of Figure 3 relative to the base unit and respective fins in accordance with the preferred embodiment; Figure 5 is a similar view to Figure 4 after swaging, wherein the rollers are omitted for clarity; Figure 6 shows an apparatus for aligning the fins prior to swaging, in accordance with the preferred embodiment; Figure 7 is a perspective view showing mounting of the rollers on an axle; Figure 8 shows a portion of the apparatus of Figure 3 for securing the base unit prior to swaging; Figure 9 is a cross-sectional side view of the apparatus of Figure 3 showing means for adjusting the axle height and details of the axle support construction; Figures 10, 11 and 12 are cross-sectional side views similar to Figure 9 which, in combination, show the passage of a base unit and fins through the apparatus of Figure 3; and Figure 13 is a perspective view showing a saw for trimming end portions of the fins which extend beyond the sides of the base unit after swaging. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to Figure 1, a heatsink is shown which has been fabricated in accordance with the prior art. In this prior art heatsink a base unit B and a plurality of fins F projecting perpendicularly therefrom, are extruded as an integral unit using well known extrusion techniques and apparatus. However, as discussed above, such prior art extrusion techniques are incapable of producing high fin density heatsinks. Turning to Figure 2, a heatsink is shown which has been fabricated using the apparatus and method of the present invention. The inventive heatsink comprises a base 1 and a plurality of fins 3 which are swaged into respective parallel grooves in the top surface of the base unit 1, as discussed in greater detail below. In the illustrated embodiment, the fins 3 are shown as having a generally serpentine profile for increasing the heat dissipating surface area. However, the fins 3 may be made flat for certain applications. Turing momentarily to Figure 4, the base unit 1 is shown in greater detail comprising a plurality of parallel generally rectangular grooves 5, each of the grooves 5 incorporating rounded corners at a lower portion thereof. It has been found that extrusions of the base portion 1 in accordance with the design of Figures 2 and 4 is both simple and inexpensive. The fin 3 is shown with reference to Figure 4 having a flanged bell-bottom lower portion. Thus, in accordance with the method of the present invention, the plurality of fins 3 and base unit 1 are separately extruded, and thereafter combined by inserting respective ones of the fins 3 into the grooves and thereafter crimping or swaging the fins 3 into the grooves for forming a dovetail connection by virtue of the respective flange portions, as discussed in greater detail below. Turning to Figure 3, in combination with Figures 4 and 5, the apparatus according to the present invention is shown for fabricating high fin density heatsinks, comprising a plurality of disks 7 mounted to an axle 9. The axle 9 is mounted for rotation in a pair of adjustable height mounting blocks 10. The disks are spaced apart a predetermined distance equivalent to the distance between successive ones of the plurality of grooves 5. The clearance between the lower most portion of the circumference of the disks 7 and the top of the working surface 11 (Figure 8) is adjusted to be slightly less than the thickness of the base unit 1. Preferably, each of the disks 7 has a width approximately equal to but no greater than the distance between successive ones of the plurality of grooves 5 minus the width of each of the fins 3. With reference to Figure 9, the working surface 11 which supports the base unit 1 while it passes beneath the disks 7 is, in turn, supported by a roller 16 mounted to a second axle 18 which is also mounted in blocks 10. The height of the axle 9, and hence the clearance between the disks 7 and working surface 11, may be adjusted by turning the handle 20 (Figure 3), which is connected to a rod 22 having a pair of geared wheels 24 (only one such wheel being shown in Figures 3 and 9). The wheels 24 engage respective cooperatively shaped wheels 26 each mounted to a further rod 28 (only one such wheel and rod being shown in Figures 3 and 9). Each rod 28 has, at an end opposite to the geared wheel 26, a threaded portion 29 (Figure 3) which engages a gear wheel 30 of a threaded bolt member 32. Upon turning the handle 20, the rod 22 rotates, causing rods 28 to rotate as a result of interaction between geared wheels 24 and 26. This, in turn, causes the gear 30 and threaded bolt 32 to rotate within mounting block 20. The axle 9 is housed within a riser block 34 which has internal threads for receiving the threaded bolt 32. Thus, in response to rotation of the threaded bolt 32, the riser block 34 may be raised or lowered. According to the preferred embodiment, one rotation of handle 20 results in an axle height adjustment of approximately 0.0635mm (2.5 x 10 -3 inches). Also shown in Figure 9 is a pair of pneumatically controlled rollers 36 and 38 which are adapted to rest on top of the fins 3 as they pass through disks 7. The rollers 36 are required to securely hold down the fins 3 in the grooves 5 before pressure is applied during swaging. The rollers 38 are required to prevent the fins 3 from rolling upwardly and around the disks 7 after swaging. In operation, the fins are loosely placed in the grooves 5 of the base unit 1. The fins 3 are then aligned relative to the spaces between respective disks 7 by means of the alignment tool 12 shown in Figure 6. This tool comprises a plurality of teeth 14 spaced at equivalent intervals to the disks 7. In Figure 6, the non-aligned positions of the fins 3 are shown in phantom. The loosely assembled unit is then advanced into the apparatus of Figure 3 by means of a pneumatic drive 16, such that respective ones of the fins 3 pass between respective disks 7, as shown in Figure 4. As the loosely assembled unit is advanced in the direction of arrow A in Figures 10, 11 and 12, the disks rotate in the direction of arrow B and the roller 16 rotates in the direction of arrow C, such that the lower most circumference portions of the disks 7 apply pressure to the regions of base unit 1 intermediate respective ones of the fins 3 for swaging the metal material in these regions to thereby crimp the fins 3 firmly in place. The pressure provided by the apparatus of Figure 3 on block 1 may be adjusted by careful selection of the height of the axle 9. The base unit 1 is held in place on working surface 11 by a pair of positioning bars 40 and 42 which are shown in detail with reference to Figure 8. A pair of toothed side retention members 44 and 46 project upwardly from the working surface 11, and are adapted to receive cooperatively shaped toothed end portions of the positioning bars 40 and 42. Thus, the base unit 1, with fins 3 is held firmly in place on the working surface 11 while the fins 3 are swaged into the base unit 1 under pressure of disks 7. Turning to Figure 7, each of the disks is shown comprising a raised circular hub portion 50 of predetermined thickness against which an adjacent one of the plurality of disks rests. The predetermined thickness of the raised circular hub portion defines the distance between respective ones of the disks 7. Each raised circular hub 50 has a keyway in the form of a rectangular key 52 therein for receiving a cooperatively shaped rectangular key 54 projecting from the axle 9, in order to mount the disks 7 for rotation in conjunction with axle 9. The dovetail connection between the groove 5 and bell-bottom shaped fin 3 of the present invention, in conjunction with the use of disk 7 applying pressure vertically downward on base unit 1, results in the fins 3 being pushed downwardly by the displaced base unit material (shown in phantom with reference to Figure 5), and into tight engagement with the corresponding grooves 5. As a result of the swaging operation according to the present invention, the fins 3 often become displaced laterally with reference to the base unit 1. Therefore, according to the present invention, as shown in Figure 13, the heatsink is inverted and secured in a mounting device 60 and a rotary blade 62 is used to cut through the excess portions of the fins 3 for trimming such excess portions off of the heatsink, in a straight forward manner. In summary, the method and apparatus of the present invention result in low cost fabrication of high fin ratio heatsinks, (eg. 6:1, 8:1, 10:1 and greater). The heatsinks are characterized by high thermal contact between the base unit and fins with no moisture or air therebetween, thus preventing corrosion and permitting the part to be anodized.

Claims: Apparatus for swaging a base unit of predetermined thickness and having a plurality of grooves therein for receiving respective bell-bottom shaped fins to form a high fin density heatsink, said apparatus comprising: a) a planar work surface; b) an axle mounted parallel to said planar work surface; c) a plurality of disks mounted for rotation about said axle and spaced apart a sufficient distance to allow respective ones of said fins to pass therebetween; d) means for adjusting the height of said axle to provide a clearance between the circumference of said disks and said planar work surface which is less than said predetermined thickness of said base unit, such that upon advancing said base unit though said clearance respective ones of said disks apply downward and horizontal pressure to said base unit intermediate respective ones of said fins for swaging said base unit so as to urge said fins downwardly into respective ones of said grooves thereby creating a secure connection between said base unit and said fins with good thermal contact therebetween. The apparatus of claim 1 wherein said sufficient distance between successive ones of said disks is equivalent to the distance between successive ones of said plurality of grooves. The apparatus of claim 2 wherein each of said disks has a width approximately equal to and no greater than said distance between successive ones of said plurality of grooves minus the width of each said fins. The apparatus of claim 1, 2 or 3, wherein each of said plurality of disks further comprises a raised circular hub portion of predetermined thickness against which an adjacent one of said plurality of disks rests, said predetermined thickness defining said sufficient distance of spacing between respective ones of said disks. The apparatus of claim 4, wherein said raised circular hub has a keyway in the form of a rectangular slot therein for receiving a cooperatively shaped key projecting from said axle for mounting said plurality of disks thereto. The apparatus of any preceding claim, wherein said means for adjusting the height of said axle further comprises a pair of threaded mounting blocks for receiving said axle, gear means connected to each of said threaded mounting blocks, and a handle connected to said gear means such that upon turning said handle said gear means causes each of said threaded mounting blocks to move vertically for raising and lowering said axle. A method of forming a high fin density heatsink from a plurality of fins each having a bottom portion of generally bell-bottom shape and a base unit of predetermined thickness having a plurality of grooves therein, comprising the steps of: (a) placing said plurality of fins loosely in respective ones of said grooves; and (b) applying downward and horizontal pressure to said base unit intermediate respective ones of said fins for swaging said base unit so as to urge said fins downwardly into said respective ones of said grooves thereby creating a secure connection between said base unit and said fins with good thermal contact therebetween.

Inventors: LIPINSKI JOSEPH, LIPINSKI, JOSEPH
Applicants: THETA R INC, R-THETA INC.
Application year: 1994
Main classification: H01L21
IPC codes: H01L21, F28F3